1. Field of the Invention
The present invention relates to an optical transmission system using an optical fiber cable as a transmission line.
2. Description of the Related Art
Optical transmission systems have been developed for large transmission capacity and long span transmission. For accomplishing the large transmission capacity, the increase of bit rate and wavelength division multiplexing system have been studied. For accomplishing the long span transmission, optical amplifiers are used. The optical amplifiers are categorized as for example a post-amplifier that raises the transmission power, a pre-amplifier that raises the sensitivity of received power, and an in-line amplifier that functions as a repeater. These optical amplifiers have been developed as products. With the optical amplifiers, the difference of levels of a received signal and a transmitted signal becomes large and the allowable loss of an optical fiber cable becomes large.
On the other hand, with optical amplifiers, the optical input level to the optical fiber cable becomes high. Thus, a new problem called non-linear effect has taken place. As an example, when the level of an optical signal that is input to the optical fiber cable is large (for example, +8 dBm for a dispersion shifted optical fiber cable and +10 dBm or more for a single mode optical fiber cable), a frequency (wavelength) shift takes place at a leading edge and a trailing edge of a pulse of the optical signal due to the optical Kerr effect (the refractive index varies depending on the intensity of light) (this phenomenon is referred to as self phase modulation). In this case, even if the spectral width of an optical signal before the transmission is narrow, the spectral width becomes wide through transmission. In addition, due to the influence of the dispersion of the optical signal on the transmission line, the waveform of the received signal becomes degraded. In other words, the upper limit of the power of transmission optical signal depends on such an influence.
Moreover, since the velocity of light that propagates in an optical fiber cable depends on the wavelength thereof, after an optical pulse with a particular wavelength is transmitted through an optical fiber cable, the pulse width may be expanded or compressed. This phenomenon is referred to as chromatic dispersion in optical fiber cable. Thus, after an optical signal is transmitted through an optical fiber cable of an optical transmission system, the waveform of the received signal varies due to the chromatic dispersion. Depending on the degree of the chromatic dispersion, a transmission error takes place. Thus, the transmission distance may be restricted due to the chromatic dispersion.
So far, transmission deterioration due to the chromatic dispersion on the optical fiber cable is prevented using a light source with a narrow wavelength width. However, in recent years, due to high bit rate of 10 Gb/s and the non-linear effect of an optical fiber cable, the transmission deterioration cannot be prevented using a light source with a narrow wavelength width.
To solve such a problem, an optical transmission system with a dispersion compensation has been used. However, since the cost of the dispersion compensator is high and the dispersion compensation amount varies corresponding to the transmission distance, there need be a variety of products. Thus, it is difficult to use the optical transmission system with a dispersion compensator.
In such a conventional technology, as a pre-chirp of the transmitter, a blue chirp (chirping parameter α<0) is used. In addition, a dispersion compensator is placed on the receiver side (between a pre-amplifier and an optical-electrical signal converter (O/E)) (namely, post compensation is performed). However, in this system, since the compensation is inflexibly performed, the loss of the dispersion compensator becomes large. The loss cannot be ignored when the transmission distance becomes long. In addition, since the input level of the optical signal becomes low, the receiver sensitivity degrades. Moreover, since the tolerance of the dispersion compensation amount for proper transmission characteristics is narrow, dispersion compensators should be prepared corresponding to the transmission distance. Thus, many types of products should be prepared. To solve such a problem, a system in which a red chirp (chirping parameter α>0) is used as a pre-chirp on the transmitter side and dispersion compensators are disposed on both the transmitter side and the receiver side was considered. FIG. 1 shows a basic structure of this system.
FIG. 1 is a block diagram showing an outlined structure of a conventional optical transmission system.
The optical transmission system shown in FIG. 1 comprises a transmitter 160, a transmission line 164 (composed of an optical fiber cable), and a receiver 165. The transmitter 160 comprises an E/O (electric-optical signal converter) 161, a dispersion compensator 162, and a post-amplifier 163. The E/O 161 converts an electric signal into an NRZ coded optical signal. The post-amplifier 163 amplifies the optical signal and sends the resultant signal to the transmission line 164. The receiver 165 comprises a pre-amplifier 166, a dispersion compensator 167, and an O/E (optical-electric signal converter) 168. The pre-amplifier 166 amplifies weakened light that has been transmitted through the transmission line 164. The dispersion compensator 167 compensates for the dispersion of the optical signal that has been transmitted through the transmission line 164. The O/E 168 converts an optical signal into an electric signal.
In the conventional optical transmission system, the transmitter 160 red-chirps an optical signal as a pre-chirp. In addition, the transmitter 160 uses an NRZ coded signal as an optical signal. The dispersion compensator 162 in the transmitter 160 compensates for a predetermined dispersion amount of an optical signal so as to cancel the dispersion of the optical signal propagated on the transmission line 164. The post-amplifier 163 amplifies the intensity of an optical signal so that it can be transmitted for a long distance.
The pre-amplifier 166 in the receiver 165 amplifies a weakened optical signal propagated on the transmission line 164 so that the optical signal can be detected. The dispersion compensator 167 adjusts the dispersion compensation amount corresponding to a dispersion amount of the transmission line 164 detected by the receiver 165 so that the receiver 165 can correctly detect the optical signal. Thus, the dispersion compensator 167 in the receiver 165 can adjust the dispersion compensation amount. The O/E 168 converts an optical signal into an electric signal. The O/E 168 sends the received signal to an electric signal processing unit (not shown) disposed downstream thereof so as to demodulate the electric signal and extract data from the optical signal.
Thus, in the system shown in FIG. 1, the transmitter 160 red-chirps an optical signal. In addition, both the transmitter 160 and the receiver 165 have respective dispersion compensators.
In this compensation system, the dispersion compensation on the transmitter side is effective. The transmitter compresses pulses corresponding to the chirping and the characteristics of the dispersion compensator. Thus, inter-symbol interference due to the increase of the pulse width on the transmission line is alleviated. In addition, since the red-chirping is used, the influence of the non-linear effect (SPM) on the transmission line is canceled. Thus, the deterioration of the waveform of the transmission signal is smaller than that in the case of the blue-chirping. Thus, since the tolerance of the compensation amount is wide, the number of types of dispersion compensators can be reduced.
However, as a problem of the system, since the dispersion compensation amount is large, many dispersion compensation optical fiber cables that are expensive should be used. Thus, the cost of the system becomes high. In addition, since the transmitter and the receiver require respective dispersion compensators, the size of the system becomes large.